1. Field of the Invention
The present invention relates to a protective element and a protective circuit for protecting a circuit to be protected from an overcurrent or an overvoltage by tripping of a PTC element.
2. Description of the Related Art
Conventionally, current fuses that are made of a low-melting-point metal, such as lead, tin or antimony, which are heated and melted off by overcurrents, are known as protective elements for interrupting overcurrents to circuits to be protected.
Moreover, PTC (Positive Temperature Coefficient) elements are known as devices, which are heated by overcurrents, whereby their resistance is increased, and the current flowing through the circuit to be protected is curbed.
Furthermore, protective devices have been proposed, in which a protective element is combined with a voltage sensing means, and the circuit to be protected is protected not only from overcurrent but also from overvoltages.
FIG. 6 is a circuit diagram of such a protective device, and FIG. 7A is a plane view and FIG. 7B is a cross-sectional view of the protective element 20X used in this circuit (see Japanese Patent Application Laid-Open No.8-236305). In the circuit in FIG. 6, the terminals A1 and A2 are connected to the electrode terminals of the device to be protected, such as a lithium-ion battery, and the terminals B1 and B2 are connected to the electrode terminals of a charging device or the like.
In this circuit, a PTC element 1 and a low-melting-point metal member 2 are connected in series between a first terminal a and a second terminal b, a heat-generating member 3 is connected between their connection point (electrode 5d) and a third terminal c, and the low-melting-point metal member 2 and the heat-generating member 3 are arranged in proximity to each other. Thus, the PTC element 1, the low-melting-point metal member 2 and the heat-generating member 3 constitute one protective element 20X.
In the protective element 20X, electrodes 5a, 5b, 5c and 5d are formed on a substrate 4, as shown in FIGS. 7A and 7B, the heat-generating member 3 is formed between the electrode 5c and the electrode 5d, the heat-generating member 3 is covered with an insulating layer 6, the PTC element 1 is formed on the electrode 5a, the low-melting-point metal member 2 is formed so as to bridge the PTC element 1, the electrode 5b and the electrode 5d on the substrate 4, and these are covered by a protective cap 7.
Moreover, this protective device is provided with a Zehner diode and a transistor as a voltage sensing means and a switching means.
With this protective device, when current is conducted to the heat-generating member 3 under regular operation, the PTC element 1 and the low-melting-point metal member 2 form a conduction line between the electrode terminals of the device to be protected, such as a lithium-ion battery, and the electrode terminals of the charging device or the like. However, when a reverse voltage that is larger than a predetermined breakdown voltage is applied to the Zehner diode, an abrupt base current ib flows, which causes a large collector current ic to flow through the heat-generating member 3 and heat the heat-generating member 3, the low-melting-point metal member 2 that is at a position near the heat-generating member 3 melts off, and the progression of an overcurrent in the device to be protected, such as a lithium-ion battery, which is connected to the terminals A1 and A2 can be prevented.
On the other hand, when an overcurrent exceeding a certain value flows through the terminals A1 and B1, first, the PTC element 1 curbs the current, then the low-melting-point metal member 2 melts off and interrupts the current.
FIG. 8 is a circuit diagram of still another protective device, and FIG. 9A is a front view and FIG. 9B is a side view of the protective element 20Y used in this circuit (see Japanese Patent Application Laid-open No. 10-98829). In this circuit, a first PTC element 1a and a second PTC element 1b are arranged on opposite sides of a stainless-steel plate 8 of the protective element 20Y. The first and the second PTC element 1a and 1b are elastically clamped by springs 9a and 9b, and assembled into a polymer protective casing 7. Moreover, as in the circuit in FIG. 6 above, a Zehner diode and a transistor are provided as a voltage sensing means and a switching means.
Consequently, with the protective circuit of FIG. 8, only the first PTC element 1a is conducted during regular operation, and during an overvoltage, current flows abruptly through the second PTC element 1b to heat the second PTC element, this heat is quickly conducted to the first PTC element, and the first PTC element trips, whereby the current into the circuit to be protected is curbed.
However, in a protective element using the above-described PTC element, to operate a PTC element 1 or 1a (FIGS. 6 and 8) that curbs the current in a conductive line in case of abnormal stances, a heat-generating member 3 (FIG. 6) or a second PTC element 1b (FIG. 8) have to be used separately.
Furthermore, to configure the protective elements 20X and 20Y using the PTC elements 1, 1a, and 1b or the heat-generating member 3, a substrate 4 or a casing 7 becomes necessary.
Therefore, there are the problems that the number of components for conventional protective elements is large and miniaturization is difficult, and manufacturing costs are high.
With regard to these problems of the prior art, it is an object of the present invention to provide a protective element that can be manufactured with fewer components, in an easy manner, and at lower cost, and can cope with overcurrent and overvoltage.
The inventors have discovered that when one PTC material is provided with three or more electrodes, the PTC material that is sandwiched by two of these operates as one separate PTC element, and thus the inventors perfected the protective element of the present invention. Moreover, they have discovered that when a circuit is configured such that two PTC elements are connected in parallel with a conduction line to a circuit to be protected, current flows through both PTC elements during regular operation, and current flows only through one of them during abnormality, then even if the current during the abnormality is comparatively smaller than during regular operation, it still trips the PTC element, and thus the protective circuit of the present invention was perfected.
In other words, the present invention provides a protective element which comprises one PTC material and at least three electrodes provided thereon, and the PTC material operates as at least two PTC elements.
Moreover, the present invention provides a protective circuit, comprising a first PTC element, a switching element and a second PTC element, wherein the first PTC element is connected in parallel with the switching element and the second PTC element connected in series; the switching element enables the conduction of current to the second PTC element under regular operation and interrupts the conduction of current to the second PTC element when a terminal voltage of the circuit to be protected is higher than a predetermined voltage; and the first PTC element trips due to the conducted current. Other objects and features of the present invention will be described or become apparent from the following disclosure.